Modulation of East Asian monsoon strength by ENSO during the warm periods of the late Holocene: Evidence from Porites corals in the northern South China Sea

As a gauge for ocean precipitation conditions, sea surface salinity (SSS) is suspected to be an excellent indicator for variations in the global hydrological cycle. The SSS conditions in the South China Sea (SCS) are closely related to precipitation changes of the East Asian monsoon. However, the characteristics and driving mechanisms of the meridional pattern of East Asian monsoon rainfall during the warm periods of the late Holocene are not well understood because of a lack of high-resolution proxies from the SCS. Here, we present three precise chronologies of monthly stable coral skeleton oxygen isotopes (delta O-18) and pair them with established composite strontium/calcium (Sr/Ca) records from the Xisha Islands to obtain a continuous reconstruction of the residual delta O-18 (Delta delta O-18, regarded as seawater delta O-18) during 1980-2007 CE (Common Era), 1149-1205 CE, and 2070-2011 a BP (years before 1950 CE). The results confirm that coral Delta delta O-18 is a good tracer for reconstructing SSS and precipitation changes in the northern SCS on seasonal and interannual timescales, with higher.d18O values implying more saline conditions and less precipitation. The mean value of fossil coral Delta delta O-18 for 1149-1205 CE was higher than that of modern coral, suggesting the existence of salty surface waters and notable dry climate episodes in the northern SCS during the late Medieval Climate Anomaly (MCA). In contrast, the average coral Delta delta O-18 for 2070-2011 a BP was significantly lighter by about 0.404% compared to that of recent decades, indicating that less saline and more humid conditions occurred in the northern SCS during the early Roman Warm Period (RWP). Synthesizing our coral records with other published precipitation reconstructions from eastern China suggested a meridional dipole spatial pattern of moisture variation over East Asia during the historical warm periods of the Holocene. During the MCA, drier conditions generally prevailed in the region south of the Yangtze River, and more humid conditions in the north. This may be closely related to the contemporaneous strong El Nino activity occurring at this time, which probably enhanced the East Asian summer monsoon (EASM) and caused a northward shift of the intertropical convergence zone (ITCZ), possibly inducing increased water vapor transport from the SCS to North China. During the RWP, the relatively weak El Nino-Southern Oscillation (ENSO) variability may weaken the intensity of the EASM and shift the ITCZ southward, possibly resulting in a spatial pattern of "wet south and dry north" in East Asia.